An important class of optical devices are those whose frequency response can be preselectively set by appropriate selection of the index of refraction profile with depth. Such devices include low pass, high pass and band pass filters, most notably among them RUGATE (holographic band rejection) filters. Heretofore such devices have been in the form of laminates, each individual layer of the laminate having a different index of refraction, these laminates being fabricated by physical vapor deposition, sputter deposition, or chemical vapor deposition techniques. Unfortunately, such laminates have several shortcomings which can be overcome only by fabricating the laminates using the extreme precautions of high substrate temperature, ultra high vacuum, and slow deposition rates. Absent these precautions, the laminates are inherently less dense than the bulk materials from which they are made, and thus have inherent porosity that facilitate infiltration by moisture or other chemical contaminants which can cause the laminates to degrade both structurally and optically over time. Because the interfaces among any laminate layers are of dissimilar materials, unless they have been carefully matched as to lattice constant and thermal expansion coefficient, there is necessarily great inherent stress at these interfaces, greatly reducing the structural soundness and durability of the laminates. Moreover, many optical devices whose fabrication is difficult if done in laminate form, could be readily constructed using a unitary material whose index of refraction could be preselectively and continuously varied as a function of the device's thickness (such as graded index antireflective coatings, and rugates).
In infrared and near infrared frequencies, both silicon (Si) and silicon nitride (Si.sub.3 N.sub.4) have been used in abrasion resistant optical devices of the laminate kind discussed above. Silicon is useful only at frequencies corresponding to over one micrometer wavelength because of silicon's characteristic bandgap at 1.1 micrometers. Stoichiometric silicon nitride is also useful at these frequencies, as well as at somewhat higher frequencies (inclusive up to the blue portion of the visible spectrum), because of its larger bandgap. Substoichiometric combinations of silicon and nitrogen (Si.sub.l-x N.sub.x) have, as x varies from 0 (pure silicon) to 4/7 (stoichiometric silicon nitride), widely varying indicies of refraction in the infrared and near infrared of 3.9 to 1.9. Thus if one could accurately control as a function of specimen thickness, the relative proportions of silicon and nitrogen in silicon nitride alloy, one could produce a unitary specimen whose optical characteristics could heretofore be approximated only with laminates.
Silicon nitride is conventionally fabricated by flowing SiH.sub.4 and NH.sub.3 onto a heated substrate in a controlled vacuum (chemical vapor deposition). The heated substrate provides the energy necessary to break hydride bonds and form the bonds between silicon and nitrogen, the extremely low background pressure and high purity feedgasses excluding contaminants such as carbon and oxygen. By requiring the use of highly heated substrates, one severely limits the materials that can be used as substrates to those having exceptionally good refractory characteristics. By fabricating silicon nitride with hydrogen containing molecules, one virtually assures that hydrogen contaminants will be incorporated during the forming of a silicon nitride specimen, degrading its characteristics.